The present invention provides a method for session changing. The method for session changing is designed for session changing during interworking between a first UE and a second UE in a packet network, and includes: reserving, during setup of an initial session between the first UE and the second UE, an interworking control function for logically controlling the interworking on a path of the initial session; and performing, when the interworking control function receives a subsequent session change request, the corresponding session changing operation according to contents of the subsequent session change request. The invention further provides a device for session changing and a packet network system with session changing function.